Andalusite | NOVA Mineralogy

Andalusite is a polymorph of kyanite and sillimanite. These three minerals share the same formula, but are stable at different P/T conditions.

Physical Properties
Chemical formula	Al₂SiO₅
Class	Nesosilicate Isolated tetrahedra
Crystal system	Orthorhombic
Habit	Stubby to elongate square prisms Individual crystals may be rounded Massive Granular
Color	Brown to red
Hardness	7.5
Specific gravity	3.18
Cleavage	Rarely seen, good {110}, poor (100)
Fracture	Subconchoidal
Luster	Vitreous
Transparency	Transparent to translucent
Streak	White
Optical Properties
PPL	Usually colorless May be weakly colored and pleochroic
XPL	1^st order yellows (pale tan) May be euhedral with a square outline and penetration twins or cross pattern
δ	0.010
Twinning	Penetration twins or Maltese cross pattern
	after Perkins, 345